A major source of methane is natural gas. Natural gas at the wellhead typically contains about 40-95% methane depending on the particular source. Other constituents include about 10% ethane with the balance being made up of CO.sub.2 and smaller amounts of propane, the butanes, the pentanes, nitrogen, etc.
Primary sources for natural gas are the porous reservoirs generally associated with crude oil reserves. From these sources come most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter. Natural gas obtained from these sources is generally utilized as a fuel at the site.
Prior to commercial use, wellhead natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, the butanes, the pentanes, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 70% to more than about 95% by volume of methane. Natural gas is used principally as a source of heat in residential, commercial and industrial service.
Most processed natural gas is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decrease, new sources that are more distant require additional transportation. Many of these distant sources are not, however, amenable to transport by pipeline. For example, sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water are not amenable to transport by pipeline. This problem has been addressed in several ways. One such solution has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited as the natural gas can be used only for one product, preempting other feasible uses. Another approach has been to liquefy the natural gas using cryogenic techniques and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by such cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas at a temperature of about -162.degree. C., transporting the gas, and revaporizing it are complex and energy intensive.
Pyrolysis processes involving the conversion of methane to higher molecular weight hydrocarbons at high temperatures, in excess of about 1200.degree. C., are known. These processes are, however, energy intensive and have not been developed to the point where high yields are obtained even with the use of catalysts. Some catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
U.S. Pat. No. 4,507,517 and U.K. Patent Application GB 2 148 935A disclose catalytic processes for converting methane to C.sub.2.sup.+ hydrocarbons, particularly hydrocarbons rich in ethylene and/or benzene, at temperatures in excess of 1000.degree. C. and high gas hourly space velocities greater than 3200 hr.sup.-1. The process disclosed in the '517 patent uses a boron compound containing catalyst. The process disclosed in the U.K. application uses a catalyst containing a metal compound of the Group IA, IIA, IIIA, IVB or Actinide series metals.
Low temperature pyrolysis (e.g., to 250.degree. C. and 500.degree. C.) of hydrocarbon feedstocks to higher molecular weight hydrocarbons is described in U.S. Pat. Nos. 4,433,192; 4,497,970; and 4,513,164. The processes described in these patents utilize heterogeneous systems and solid acid catalysts. In addition to the solid acid catalysts, the reaction mixtures described in the '970 and '164 patents include oxidizing agents. Among the oxidizing agents disclosed are air, O.sub.2 -O.sub.3 mixtures, S, Se, SO.sub.3, N.sub.2 O, NO, NO.sub.3, F, etc.
The catalytic oxidative coupling of methane at atmospheric pressure and temperatures of from about 500.degree. C. to 1000.degree. C. has been investigated by G. E. Keller and M. M. Bhasin. These researchers reported the synthesis of ethylene via oxidative coupling of methane over a wide variety of metal oxides supported on an alpha alumina structure in Journal of Catalysis, 73, 9-19 (1982). This article discloses the use of single component oxide catalysts that exhibited methane conversion to higher order hydrocarbons at rates no greater than 4%. The process by which Keller and Bhasin oxidized methane was cyclic, varying the feed composition between methane, nitrogen and air (oxygen) to obtain higher selectivities.
Methods for converting methane to higher molecular weight hydrocarbons at temperatures in the range of about 500.degree. C. to about 1000.degree. C. are also disclosed in U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; 4,443,649; and 4,523,049.
U.S. Pat. Nos. 4,172,810; 4,205,194; and 4,239,658 disclose the production of hydrocarbons including ethylene, ethane, propane, benzene and the like, in the presence of a catalyst-reagent composition which comprises: (1) a group VIII noble metal having an atomic number of 45 or greater, nickel, or a group Ib noble metal having an atomic number of 47 or greater; (2) a group VIb metal oxide which is capable of being reduced to a lower oxide; and (3) a group IIa metal selected from the group consisting of magnesium and strontium composited with a passivated, spinel-coated refractory support or calcium composited with a passivated, non-zinc containing spinel-coated refractory support. The feed streams used in the processes disclosed in these patents do not contain oxygen. Oxygen is avoided for the purposes of avoiding the formation of coke in the catalyst. Oxygen is generated for the reaction from the catalyst; thus periodic regenerations of the catalysts are required.
U.S. Pat. No. 4,450,310 discloses a methane conversion process for the production of olefins and hydrogen comprising contacting methane in the absence of oxygen and in the absence of water at a reaction temperature of at least 500.degree. C. with a catalyst comprising the mixed oxides of a first metal selected from lithium, sodium, potassium, rubidium, cesium and mixtures thereof, a second metal selected from beryllium, magnesium, calcium, strontium, barium, and mixtures thereof, and optionally a promoter metal selected from copper, rhenium, tungsten, zirconium, rhodium, and mixtures thereof.
U.S. Pat. No. 4,560,821 discloses a continuous method for synthesizing hydrocarbons from a methane source which comprises contacting methane with particles comprising an oxidative synthesizing agent under synthesis conditions wherein particles recirculate between two physically separate zones: a methane contact zone and an oxygen contact zone. These particles are maintained in each of the two zones as fluidized beds of solids. The oxidative synthesizing agents are reducible oxides of metals selected from the group consisting of Mn, Sn, In, Ge, Pb, Sb and Bi.
PCT International Application No. PCT/GB85/00141 discloses a process for the production of synthesis gas and higher molecular weight hydrocarbons in which a saturated hydrocarbon and an oxygen containing gas having a ratio of hydrocarbon to oxygen of greater than the stoichiometric ratio for complete combustion are introduced into a bed of an inert particulate material, the upward flow rate of the hydrocarbon/oxygen containing gas stream being sufficient to fluidize or to produce a spouting action of the bed material, whereby at least a part of the particulate material is thrown up above the bed surface and subsequently falls back into the bed, the hydrocarbon and oxygen containing gas being ignited and reacted together, and the products of the reaction being withdrawn.